(a) Field of the Invention
This invention relates to an electroluminescent panel for use in an input/output device of a computer or the like to display an image, such as alphanumeric symbols, a static picture, a motion picture, and the like and to a method of manufacturing the electroluminescent panel.
(b) Description of Prior Art
A conventional electroluminescent panel of the type described comprises a transparent substrate, a transparent electrode on the substrate, a back electrode opposite to the transparent electrode, and an electroluminescent layer placed between the transparent and the back electrodes. In addition, a dielectric layer is usually interposed between the transparent electrode and the electroluminescent layer.
With this structure, electroluminescent light is emitted from the electroluminescent layer and can be seen through the transparent substrate in a well-known manner when an a.c. voltage is supplied between the transparent and the back electrodes.
The dielectric layer is useful for raising a breakdown voltage of the electroluminescent panel.
In order to raise the breakdown voltage, it is preferable that the dielectric layer has a high dielectric strength, a high relative dielectric constant, and a low dielectric loss.
Heretofore, various oxides are used as materials of the dielectric layer. Such oxides are, for example, yttrium oxide (Y.sub.2 O.sub.3), tantalum pentoxide (Ta.sub.2 O.sub.5), aluminum oxide (Al.sub.2 O.sub.3), zirconium oxide (ZrO.sub.2), hafnium oxide (HfO.sub.2), lead titanate (PbTiO.sub.3), and barium tantalate (BaTa.sub.2 O.sub.6).
On manufacturing an electroluminescent panel of the above-mentioned type, the dielectric layer is generally formed on the transparent electrode by the use of a sputtering technique in order to prevent occurrence of fine defects. Then, the electroluminescent layer is formed on the dielectric layer and is subjected to a heat treatment at a temperature between 400.degree. C. and 600.degree. C. so as to activate the electroluminescent layer.
It has been found that the above-mentioned transparent electrode is likely to be blackened and to thereby increase an electric resistance thereof when the dielectric layer is manufactured in the manner mentioned above. Such a blackened transparent electrode results in a reduction of brightness of the electroluminescent panel. Such an increased electric resistance of the transparent electrode brings out increasing power consumption in the electroluminescent panel and also degrading panel quality. Such degradation of panel quality results from unevenness of the brightness due to variation of the electric resistance.
Alternatively, a layer of silicon nitride (Si.sub.3 N.sub.4) is often deposited as the dielectric layer on the transparent electrode in a non-oxygen atmosphere. It has been confirmed that deposition of silicon nitride is effective to protect the transparent electrode from being blackened. However, the dielectric layer of Si.sub.3 N.sub.4 is very weak in adhesion to the transparent electrode. Therefore, peeling off often occurs between the transparent electrode and the dielectric layer of Si.sub.3 N.sub.4 when the electroluminescent layer is subjected to the heat treatment. Such detachment rarely occurs even when the dielectric layer is of the oxide.
An improved electroluminescent panel is disclosed in Japanese Unexamined Patent Publication No. Syo 52-33491, namely, 33491 of 1977. The electroluminescent panel comprises an intermediate layer consisting of silicon dioxide (SiO.sub.2) between the transparent electrode and the dielectric layer of Si.sub.3 N.sub.4. The intermediate layer of SiO.sub.2 is formed on the transparent electrode by the use of sputtering so as to increase adhesion between the transparent electrode and the dielectric layer of Si.sub.3 N.sub.4. However, the electroluminescent panel is defective in that the transparent electrode is prone to be blackened and to thereby increase an electric resistance thereof during manufacturing the electroluminescent panel. Thus, a reduction of the brightness and an increase of power consumption are inevitable in the above-mentioned electroluminescent panel.
Another improved electroluminescent panel is disclosed by Etsuo Mizukami et al in U.S. Pat. No. 4,188,565. The electroluminescent panel comprises dielectric layer of silicon-oxynitride between the transparent electrode and the electroluminescent layer. The dielectric layer of silicon-oxynitride is deposited on the transparent electrode by the use of sputtering. The sputtering is carried out by the use of a target of silicon in the presence of oxygen in addition to nitrogen. The dielectric layer of silicon-oxynitride may bring about a good adhesion to the electroluminescent layer. However, the dielectric layer of silicon-oxynitride may be poor in adhesion to the transparent electrode. In addition, a reduction of brightness and an increase of power consumption are unescapable even by depositing the dielectric layer of silicon-oxynitride. Therefore, adhesion of the transparent electrode to the dielectric layer of silicon-oxynitride is not always sufficient to avoid detachment of the transparent electrode from the dielectric layer.